Atmospheric air filtration unit, air pre-filtration unit, and associated air filtration system for removeable attachment thereof

ABSTRACT

An air filtration system including a pre-filtration unit having a filter section and a spacer section attached to the filter section and extending downstream from the filter section, a pre-filter attachment device having a pre-filter securing section and a first plurality of coupling members, and a filtration unit. The filter section includes a pre-filter. The spacer section includes an at least partially open end located on a downstream side of the spacer section to expose a downstream face of the pre-filter. The filtration unit includes a filter frame, an upper section thereof including an upstream-facing surface having a plurality of openings, a filter secured inside the lower section of filter frame, and a second plurality of coupling members located on the filter frame and coupleable with the first plurality of coupling members of the pre-filter attachment device to removeably secure the pre-filtration unit to the filtration unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of and claims thebenefit of priority under 35 U.S.C. §120 from U.S. application Ser. No.12/839,199, filed Jul. 19, 2010, the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention is atmospheric air filtration units,and in particular, an air filtration unit having an air pre-filtrationunit and pre-filter attachment device associated therewith for abuttingthe air pre-filtration unit to the air filtration unit.

3. Background

In heating, ventilating, and air conditioning systems (HVAC systems),and more specifically in variable air volume (VAV) HVAC systems,mini-pleat box filters can be used as medium and high efficiency airfilters. Mini-pleat box filters may be configured to filter an air flowin varying filtration efficiencies, such as 65%, 85%, 95%, and 98%. Forcompatibility with most HVAC systems, mini-pleat box filters aregenerally square or rectangularly-shaped when viewed from an upstreamside.

The fiberglass paper or synthetic non-woven filtration media used inmini-pleat box filters are conventionally rectangularly-shapedmini-pleat media packs which are placed in the mini-pleat box filters inpairs to form a V-shaped filter cells. These media packs are sometimesrigid enough that they may be used without the use of separators.Traditionally, separators held the filtering pleats in position and wereoften made of corrugated aluminum sheets. The non-woven synthetic fibermedia packs can be favored in some applications over the separator-typefiltering pleats as they can reduce the decrease in pressure (“thepressure drop”) between an upstream side of the mini-pleat box filterand a downstream side. They also can provide more air flow through thefiltration media and can expose the full media surface to the airflow.

In multiple stage filtration systems, a panel or pad filter (pre-filter)can be placed upstream of a mini-pleat box filter (main filter) as partof a pre-filtration stage to remove larger particle sizes from anairflow flowing through the HVAC system. This can be useful to prolongthe filtration life of the main filter. The pre-filter stage isgenerally located upstream from the main filter at a sufficient distanceso as to prevent the pre-filter from blinding the main filter. If thepre-filter is arranged so closely to the main filter, that it almosttouches the ribs of the main filter, it can disturb the flow of airreaching the pre-filter and the main filter, and thus prevent at least aportion of the airflow from crossing through the main filter. Thisprevention of airflow by the pre-filter, defined herein as “blinding,”can result in higher energy cost and a higher pressure drop over thetotal filter assembly and reduce the filtration capacity of the mainfilter.

Some multiple stage filtration systems provide an entirely separatesection in the filtration system to accommodate the pre-filter. As aresult, a pre-filter holding frame must be added to these filtrationsystems, and the filtration systems must be made larger as they requiremore space in order to accommodate the separate pre-filter section.These filtration systems may also require maintenance personnel tochange the main filter and the pre-filter in different locations of thefiltration system.

Other multiple stage filtration systems that combine the pre-filter andmain-filter stages, do so at the expense of filtration efficiency. Forexample, in one alternative an upstream edge of the V-shaped filtercells is recessed from an upstream face of the mini-pleat box filter toseparate the V-shaped filter cells from the filtration media of apre-filter that is abutted to the upstream face. A disadvantage of thisapproach can be in some instances that the recessing is done by reducingthe size of the V-shaped media packs, which reduces the filteringpotential of the mini-pleat box filter.

In another example, the filtration media of the pre-filter can berecessed in areas located opposite of an upstream edge of the V-shapedmedia packs. However, doing so can lower the filtering potential of thepre-filter since filtration media must be removed to accommodate therecessed areas. Additionally, in the above examples, conventional metalretainers are generally used to hold the pre-filter in place. Use ofthese retainers can be disadvantageous in certain instances as the metalretainers can fit loosely and easily corrode, producing corrosion dustparticles.

SUMMARY OF THE INVENTION

The present invention provides an air pre-filtration unit, a an airfiltration unit, and an air filtration system for the abutted removeableattachment of the air pre-filtration unit to the air filtration unitwhile maximizing the filtering potential of the air filtration unit andair pre-filtration unit, and preventing blinding of the air filtrationunit by the air pre-filtration unit. Additionally, the air filtrationsystem eliminates the need for two separate stages of filtration in anHVAC filtration system, thereby generally allowing for a reduction inenergy consumption and for a more compact and less expensive HVACsystem.

In a first separate aspect of the present invention, an air filtrationunit includes a filter frame with an upper section located on anupstream side of the air filtration unit and a lower section connectedto the upper section. The lower section is located downstream from theupper section. The upper section includes an upstream-facing surfacehaving a plurality of openings and an air filter secured inside thelower section of filter frame such that the plurality of openings of theupper section provide fluid flow access to the air filter. The airfiltration unit further includes a plurality of coupling members locatedon the filter frame, the plurality of coupling members configured toremoveably secure a pre-filter attachment device to the upstream side ofthe air filtration unit via corresponding coupling members disposed onthe pre-filter attachment device.

Further, in the first separate aspect of the present invention, theplurality of coupling members may be slotted protrusions that protrudeoutwardly from a surface of the filter frame. The plurality of couplingmembers may be made integral with the filter frame. The air filter mayinclude a plurality of V-shaped filter cells from a cross-sectionalview, where each of the V-shaped filter cells includes an open end on anupstream side to communicate with a corresponding opening of theplurality of openings of the upper section of the filter frame. Thelower section may include a first side section and a second side sectionto enclose the air filter within the filter frame, the first sidesection and the second side section each including V-shaped grooves onsides facing the air filter to hold corresponding V-shaped ends of eachof the plurality of V-shaped filter cells.

Optionally, in the first separate aspect of the present invention, theplurality of openings of the upstream-facing surface may be defined byparallelly arranged rib sections that connect opposite sides of theupper section, where the rib sections have a pointed cross-sectionalprofile in an upstream direction such that the pointed cross-sectionalprofile divides and directs fluid flowing downstream through thefiltration system into each open end of V-shaped filter cells via thecorresponding opening of the plurality of openings of theupstream-facing surface.

In a second separate aspect of the present invention, an airpre-filtration unit includes a filter section having a pre-filter, and aspacer section attached to a periphery of the filter section. The spacersection extends downstream from the filter section and has an at leastpartially open end located on a downstream side of the spacer section toexpose a downstream face of the pre-filter. Optionally, the airpre-filtration unit may have a height in an upstream direction of about4 inches, the filter section may have a height in an upstream directionof about 2 inches, and the spacer section may be defined by a downstreamportion of a frame of the pre-filter unit.

In a third separate aspect of the present invention, a filtration systemincludes the air pre-filtration unit, a pre-filter attachment devicehaving a pre-filter securing section and a first plurality of couplingmembers, and the air filtration unit as provided in the first separateaspect of the present invention. The air pre-filtration unit may be astandard air pre-filtration unit or one as provided in the secondseparate aspect of the present invention. The plurality of couplingmembers of the filter frame are coupleable with the first plurality ofcoupling members of the pre-filter attachment device to removeablysecure the pre-filter attachment device to the upstream side of the airfiltration unit. The pre-filter attachment device may removeably securethe air pre-filtration unit to the air filtration unit in an abuttedposition on the upstream side of the air filtration unit such that thedownstream face the air pre-filter retained in the air pre-filtrationunit is located at least 2 inches from the upstream-facing surface ofthe air filtration unit.

In a fourth separate aspect of the present invention, any of theforegoing aspects may be employed in combination.

Accordingly, an improved air pre-filtration unit, air filtration unit,and air filtration system are disclosed. Advantages of the improvementswill appear from the drawings and the description of the followingembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals refer to similarcomponents:

FIG. 1 illustrates an upstream-side perspective view of a filtrationunit according to a first embodiment;

FIG. 2 illustrates a downstream-side perspective view of the filtrationunit according to the first embodiment;

FIG. 3 illustrates a perspective view of a coupling device of thefiltration unit according to the first embodiment;

FIG. 4 illustrates a partial perspective view of an inside surface of aside portion of the filtration unit according to the first embodimentand a perspective view of a complementary pre-filter attachment deviceaccording to a second embodiment;

FIG. 5 illustrates another partial perspective view of a side portion ofthe filtration unit according to the first embodiment and a perspectiveview of a complementary pre-filter attachment device according to thesecond embodiment;

FIG. 6 illustrates an exploded perspective view of a filtration systemaccording to a third embodiment;

FIG. 7 illustrates a cross-sectional view of the filtration systemaccording to the third embodiment; and

FIG. 8 illustrates a cut-away perspective view of a pre-filtration unitaccording to a fourth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A pre-filtration unit, filtration unit, and filtration system areillustrated in FIGS. 1-8. Wherever possible, the same referencecharacters are used in the figures and the description to refer to thesame or like parts.

FIGS. 1 and 2 illustrate a filtration unit 10 according to a firstembodiment. The filtration unit 10 can be constructed out of moldedplastic materials, but may also be constructed out of metal or othersuitable materials. The filtration unit 10 is a mini-pleat V-bankbox-type air filter having an upstream side 19 that faces an airflowcrossing through the filtration unit 10 and a downstream side 18. Thefiltration unit 10 includes a boxlike plastic filter frame 14 and afilter made up of a plurality of V-shaped mini-pleat pack filter cells9(90 a-d). The filter frame 14 has an outer structure defined byopposing side panels 40 a and 40 b, side frame rails 11 a and 11 b, andrib sections 30 a-c. Preferably, an outer rectangular periphery of thefilter frame 14, when viewed from an upstream side, is 24 inches by 24inches, or alternatively 24 inches by 12 inches, and a depth of thefilter frame is 12 inches, although these dimensions might vary slightlyand other sizes may conceivably be used.

In this embodiment, the side panels 40 a and 40 b are identical mirrorimages of each other, W-shaped, and respectively include inner sides 45a and 45 b and outer sides 47 a and 47 b, as further shown in FIGS. 1,2, 4 and 5. A plurality of V-shaped channels 54 are provided on theinner sides 45 a and 45 b to enclose and secure V-shaped ends (notdepicted) of the V-shaped filter cells 90 a-d. The V-shaped channels 54provided on the inner sides 45 a and 45 b are connected at an upstreamend of the inner sides 45 a and 45 b to define a continuous zigzaggingchannel 55 that zigzags along W-shaped ends 46 of the side panels fromone upstream corner of the inner sides 45 a and 45 b to an oppositeupstream corner of the inner sides of the side panels 45 a and 45 b.“Upstream” is defined as describing a position located closer to asource of an airflow, and “downstream” is defined as describing aposition located further away from the source of the airflow than the“upstream” position. Similarly, when describing a location, “upper” isdefined as describing a position located upstream of a “lower” position.

The side frame rails 11 a and 11 b connect the side panels 40 a and 40 bvia locking members 52. The locking members 52 are block-shapedprotrusions provided at upstream corners of the inner sides 45 a and 45b. These block-shaped protrusions 52 interlock with correspondingrectangularly-shaped hollow ends (not depicted) of the side frame rails11 a and 11 b to form a boxlike frame shape.

The rib sections 30 a-c are equidistantly spaced from one another andthe from side frame rails 11 a and 1 b and parallelly span from the sidepanel 40 a to the side panel 40 b. The rib sections 30 a-c respectivelyconnect the side panels 40 a and 40 b via rib brackets 41 a-c providedat the upstream peaks 56 a-c of the zigzagging channel 55, as shown inFIGS. 1 and 4. The rib brackets 41 a-c are respectively defined byhat-shaped outer locking members 58 a-c and triangular inner lockingmembers 57 a-c. The hat-shaped outer locking members 58 a-c arerespectively incorporated into the upstream peaks 56 a-c of thezigzagging channel 55. The triangularly-shaped locking members 57 a-care disposed on the inner sides 45 a and 45 b, and protrude outwardlytherefrom, in an inner portion of the hat-shaped outer locking members58 a-c, respectively. Hollow ends (not depicted) of the rib sections 30a-c are respectively inserted into the rib brackets 41 a-c such that thehat-shaped outer locking members 58 a-c and the triangular inner lockingmembers 57 a-c respectively grip outer and inner end surfaces of the ribsections 30 a-c.

Together, the side frame rails 11 a and 11 b and a flanged upstream endof the side panels 40 a and 40 b define a flanged, rectangular uppersection 13 when viewed from an upstream side of the filtration unit. Theflanged rectangular upper section 13 includes an upstream-facing surface17 and a downstream-facing surface 50. A downstream portion of sidepanels 40 a and 40 b define a lower section 15 that secures and enclosesthe V-shaped filter cells 90 a-d.

The parallel rib sections 30 a-c spanning from the side panel 40 a tothe side panel 40 b define a plurality of openings 12 a-d thatcorrespond with the open ends 92 of the V-shaped filter cells 90 a-dthat are secured inside the lower section 15. The rib sections 30 a-calso cover abutted upstream edges (not depicted) of the V-shaped filtercells 90 a-d that are provided in the filter frame 14. Outermostupstream edges (not depicted) of the outermost V-shaped filter cells 90a and 90 d that do not abut an adjacent V-shaped filter cell are coveredby half rib sections 34 a and 34 b included with the side frame rails 11a and 11 b which span across the filtration unit from the side panel 40a to the side panel 40 b. The upstream facing surfaces 33 of the ribsections 30 a-c are aerodynamically shaped to divide and direct theairflow into the corresponding openings 12 a-d.

In this embodiment, the rib sections 30 a-c covering the upstream edgesof the V-shaped filter cells 90 a-d can be substantially flush with theupstream-facing surface 17 of the filtration unit 10, such that thelargest possible V-shaped filter cells can be provided inside the filterframe 14. This can, in turn, allow more filtration media to beaccommodated in the V-shaped filter cells 90 a-d and can result in afiltration unit that is more efficient and that provides a lowerpressure drop across the filtration unit.

In one embodiment, upstream facing surfaces 33 of rib sections 30 a-cmay come to a point along a center 31 and slope downstreamwardlytherefrom to parallel edges of the abutting openings 12 a-d. Similarly,the half rib sections 34 a and 34 b slope downstreamwardly from atransition line 35 that denotes a transition betweenrectangularly-shaped outer portions 36 and the half rib sections 34 aand 34 b to outermost edges of the outermost openings 12 a and 12 d. Theaerodynamic shape of the rib sections 30 a-c and the half rib sections34 a and 34 b splits and directs the airflow entering the filter unit 10into the open ends 92 of V-shaped filter cells 90 a-d, therebydecreasing and mitigating air pressure resistance, helping to preventblinding by an attached pre-filtration unit, and decreasing energyconsumption from fans pushing the airflow.

As shown in FIGS. 1 and 2, the V-shaped filter cells 90 a-d are V-styleor V-bank filter cells formed of pairs rectangular non-woven syntheticfiber media packs or fiberglass paper packs 91 that are adjoined alongdownstream ends in a V-shape orientation by fibrous backing materialadhered to the downstream ends, where the adjoining downstream endsdefine closed ends 93 of the V-shaped filter cells 90 a-d. The upstreamends of the pairs of rectangular synthetic media or fiberglass packs 91are separated to define open ends 92 of the V-shaped filter cells 90a-d. While FIGS. 1 and 2 depict using four V-shaped filter cells 90 a-d,any number of V-shaped filter cells may conceivably be used.

As shown in FIGS. 1-4, the filtration unit 10 further includes identicalcoupling members 20 a-d incorporated into the side panels 40 a and 40 bof the filter frame 14. FIG. 3 shows only coupling member 20 a but theaspects of the coupling member 20 a in this figure are applicable to allthe coupling members 20 a-d located on both side panels 40 a and 40 b.Coupling members 20 a-d may be integrally formed on the inner sides 45 aand 45 b at upstream ends, and respectively centered in between theevenly spaced side frame rail 11 a, rib sections 30 a-c, and side framerail 11 b. The coupling members 20 a-d are not limited to being integralwith the filter frame 14, but may be separately formed and attached tothe filter frame 14 with fasteners. Also, the coupling members 20 a-dmay conceivably be placed in other locations on the frame.

The coupling members 20 a-d are preferably slots defined by opposingsides 24 a and 24 b that protrude outwardly from inner sides 45 a and 45b, where opposing sides 24 a and 24 b of the slots respectively includeidentical flanges 22 a and 22 b at protruding ends. Flanges 22 a and 22b are oriented inwardly such that they face one another, as shown byarrows A in FIG. 3.

According to a second embodiment of the present invention, asillustrated in FIGS. 4 and 5, a pre-filter attachment device 70 includesa pre-filter securing section 76 and coupling section 74 comprising aplurality of coupling members. The coupling members depicted in FIGS. 4and 5 are flat elongated protrusions 71 a-d that protrude outwardly in adownstream direction from the pre-filter securing section 76 and insertinto the slots defined by opposing sides 24 a and 24 b and the flanges22 a and 22 b of coupling members 20 a-d. Coupling members having othersuitable configurations may be also be used. The slots grip the flatelongated protrusions 71 a-d to removeably secure a pre-filterattachment device 70 to each of the side panels 40 a and 40 b. While, inthis embodiment, the pre-filter attachment device 70 is constructed frommolded plastic materials, it may also conceivably be constructed fromother suitable materials.

Further, as shown in FIGS. 4 and 5, the pre-filter securing section 76includes a flat rectangularly elongated backbone 72 and a plurality ofbrackets 78 that are sized to slideably accept and grip an edge of acorrespondingly sized pre-filtration unit. The flat elongatedprotrusions 71 a-d are formed on an inner edge of the flat rectangularbackbone 72 and protrude downstream from and perpendicular to adownstream surface of the flat rectangular backbone 72. The plurality ofbrackets 78 are L-shaped brackets formed on an outer edge of the flatrectangular backbone 72 and protrude upstream from and perpendicular toan upstream surface of the flat rectangular backbone 72. The flanges 75of the plurality of brackets 78 protrude inwardly at an upstream end ofthe brackets 78. In this embodiment, a downstream surface of the flanges75 is located about 2 inches or about 4 inches from an upstream surfaceof the flat elongated rectangular backbone 72 to respectivelyaccommodate a 2 inch or 4 inch thick pre-filtration unit, however thebrackets may be sized to accommodate the thickness of any pre-filtrationunit, up to 12 inches or more.

In a third embodiment of the present invention, as shown in FIGS. 6 and7, a filtration system 60 includes the filtration unit 10, twopre-filter attachment devices 70, one coupled to each of the side panels40 a and 40 b of the filtration unit 70 via the slotted coupling members20 a-d and the flat elongated protrusions 71 a-d, and a pre-filtrationunit 100 slideably inserted into the brackets 78 of the pre-filterattachment devices 70. When the pre-filter attachment devices 70 areremoveably attached to the filtration unit 10, the flat rectangularbackbones 72 of the pre-filter attachment devices 70 abut an edge of theupstream-facing surface 17 of the side panels 40 a and 40 b,respectively. The brackets 78 of the pre-filter attachment devices 70are sized such that the pre-filtration unit 100 may be slideablyinserted and securely held in place when used to filter an airflow, andsubsequently slideably removed when being replaced during maintenance,for example.

Pre-filtration units secured by the pre-filter attachment devices 70 maybe constructed of cardboard, plastic, galvanized metal, or othersuitable materials. The brackets 78 of the pre-filter attachment devices70 are preferably sized to accommodate standard 2 inch or 4 inch thickindustry-size pre-filtration units or a pre-filtration unit 100, asdescribed below in a fourth embodiment. However the brackets 78 are notlimited to these sizes, and may be configured to accommodatepre-filtration units of any size or shape. If it is desired that apre-filtration unit having a different thickness in an upstreamdirection be used, the existing pre-filter attachment devices may beremoved by being pulled out from the slotted coupling members 20 a-d ofthe filtration unit 10 and replaced with pre-filter attachment deviceshaving brackets that are appropriately sized to slideably accommodateand secure the pre-filtration unit having the different thickness.

FIGS. 6-8 depict a pre-filtration unit 100 according to a fourthembodiment. The pre-filtration unit 100 is a rectangular panel-typefiltration unit that includes a frame 107 divided into a filter section108 having a pleated filtration media pack 101 and a spacer section 109defining a void 103. The frame 107 of the pre-filtration unit 100 may beconstructed from cardboard, plastic, galvanized metal, or other suitablematerials. Preferably, an outer rectangular periphery of the frame 107of the pre-filtration unit 100, when viewed from an upstream side, is 24inches by 24 inches, or alternatively 24 inches by 12 inches, however,these dimensions might vary slightly, and other sizes may conceivably beused. Generally, the outer rectangular size of the frame 107 of thepre-filtration unit 100 that is slideably inserted into the pre-filterattachment device 70 is sized to match the outer rectangular size of theupstream-facing surface 17 of the filtration unit 10.

The frame 107 of the pre-filtration unit 100 has a thickness from anupstream side 106 to a downstream side 111 of about 4 inches. The frame107 includes a latticed horizontal retainer 105, horizontal beingdefined as an orientation that is perpendicular the upstream direction,that divides the frame 107 into the two sections, namely the filtersection 108, and the spacer section 109. An upstream side of the frame107 is also latticed to allow airflow to pass through the filtrationmedia pack 101. The filter section 108 is defined by the horizontalretainer 105 and the portion of the frame 107 connected to and locatedabove the horizontal retainer 105. The horizontal retainer 105 retainsand exposes a downstream face 110 of the filtration media pack 101located inside the filter section 108. The filter section 108 is sizedto accommodate the rectangular pleated filtration media pack 101 havinga thickness of about 2 inches in an upstream direction. Depending on theapplication, however, other thicknesses of the frame 107 and thefiltration media pack 101 may conceivably be used.

The spacer section 109 is located downstream of the filter section 108and is defined by the downstream side 111 and the portion of the frame107 located below the horizontal retainer 105. The downstream side 111is latticed similar to the upstream side 106 and the horizontal retainer105 to define at least a partially open downstream end that, incombination with the openings of the latticed horizontal retainer 105,exposes the downstream face 110 of the filtration media pack 101 in thedownstream direction. The upstream side 106 and the downstream side 111are not limited to being latticed but may be open or may have othertypes of openings formed therein. Similarly, the horizontal retainer 105may be configured in other suitable ways that both retain and expose thedownstream face 110 of the filtration media pack 101 as described above.

As shown in FIG. 6, when the pre-filtration unit 100 is slideablyinserted into the pre-filter attachment devices 70 which are already orsubsequently coupled to the side panels of 40 a and 40 b filtration unit10 via the coupling members 20 a-d and 71 a-d, the spacer section 109spaces the downstream face 110 of the filtration media pack 101 retainedby the horizontal retainer 105 at least two inches away from theupstream-facing surface 17 of the filtration unit 10 in an upstreamdirection. As discussed above, spacing the downstream face of thefiltration media 101 retained by the horizontal retainer 105 at leasttwo inches away from the upstream-facing surface 17 of the filtrationunit 10, helps to prevent blinding of the filtration unit 10 by thepre-filtration unit 100. Additionally, the efficiency of thepre-filtration unit 100 can be maximized as there are no recessesrequired to be formed in the filtration media pack. Further, whencombined, the aerodynamic rib sections 30 a-c and 34 a and 34 b, thepre-filter attachment device 70, and the spacer section 109 provide afiltration system that has a more balanced and lower pressure drop, thussaving considerable money running the HVAC system. Also, running theHVAC system with lower pressure drops provides savings of up to 40% inenergy consumption.

Therefore, a pre-filtration unit, filtration unit, and filtration systemare disclosed. While embodiments of this invention have been shown anddescribed, it will be apparent to those skilled in the art that manymore modifications are possible without departing from the inventiveconcepts herein. The invention, therefore, is not to be restrictedexcept in the spirit of the following claims.

1. An air pre-filtration unit comprising: a filter section located on anupstream side of the air pre-filtration unit and including an airpre-filter, and a spacer section attached to the filter section andextending downstream from the filter section, the spacer sectionincluding an at least partially open end located on a downstream side ofthe spacer section to expose a downstream face of the air pre-filter. 2.The air pre-filtration unit according to claim 1, wherein the airpre-filtration unit has a thickness in an upstream direction of about 4inches, the filter section has a thickness in an upstream direction ofabout 2 inches, and the spacer section is defined by a downstreamportion of a frame of the air pre-filtration unit.